This invention relates to an apparatus and method of detecting the presence of liquids, vapors and gases. More specifically, this invention relates to an apparatus capable of detecting hydrocarbon liquids and vapors from the change in the resistivity of a detector sensor exposed to those materials.
For widespread commercial use, a particular requirement is a detector's ability to sense hydrocarbons, such as gasoline, leaking from an underground tank into the environment through a change in resistivity of the detector element as it absorbs the hydrocarbons. It is also a requirement to have a detector which operates at ambient temperature, responds quantitatively and reversibly after many exposures, responds over a wide hydrocarbon concentration range, and responds to both liquid and vaporous hydrocarbons. A particular added requirement is a detector sensor with the ability to achieve increased sensitivity by concentrating the hydrocarbons from the environment into the detector, for example, by absorption. Of additional importance is a detector sensor which has the ability to vary its resistance in proportion to the amount of hydrocarbons in its environment.
As contamination of ground water supplies and the detection of noxious vapors in the environment become an increasing concern of the public and subject to greater regulation, it is increasingly important to have a simple straight forward and cost-effective apparatus for detecting these contaminants. The apparatus should be cost-effective, sensitive to low concentrations of the contaminant, and capable of making repeated or continuous monitorings. The device should also have low maintenance and preferably estimate the quantity of hydrocarbons that it is exposed to by the magnitude of change of its response. It would also be desirable if the detector sensor can be retrofitted into existing devices, be immune to false readings and differentiate leaks from naturally occurring background contamination.
Commercial hydrocarbon leak detectors have employed various types of the detection methods and technologies. Thermal conductivity detectors provide continuous monitoring but are high in cost and may require to about 1/4 inch of the hydrocarbon on a water surface for detection. Flame combustion detectors are also costly but have the advantage of being contamination proof. However, flame combustion detectors require an energy source such as propane fuel in order to operate and also require frequent maintenance. Catalytic combustion detectors are low in cost and offer high sensitivity. However, they require an energy source to operate at their required elevated temperatures. At these temperatures, the fine wires in the detector are susceptible to corrosion, e.g., from sulfur compounds found in gasoline. The catalyst can be poisoned by additives in gasoline, e.g., lead alkyls. Metal oxide semi-conductor (MOS) detectors are somewhat lower in cost, but they also require an energy source to operate at an elevated temperature. At these elevated temperatures, the fine wires contained in the detector are also susceptible to corrosion. These MOS detectors are undesirably affected by moisture and suffer from drift in base line sensitivity. Carbon detectors are low in cost, but they are undesirably affected by humidity, are mechanically unstable, and are prone to continuous loss of sensitivity. Also, once exposed, they become contaminated and tend not to be reversible. Some carbon detectors require an energy source to operate at an elevated temperature.
The initial detector performs best when detecting liquids while the latter four detectors are primarily useful in detecting vapors. Thus, it would be highly desirable to have a detector sensor and a complete detector which can function adequately in the presence of both contaminant vapors or liquids.
Another approach for a combination gas, vapor, or liquid detector was disclosed by James P. Dolan et al in U.S. Pat. No. 3,045,198. The sensitivity of this apparatus was subsequently improved by operating the device in a non-linear region or in a current saturated mode with an improved detector element as disclosed in U.S. Pat. Nos. 4,237,721; 4,224,595; and 4,129,030. All of these patents are incorporated herein by reference for all purposes.
According to the teachings of the patents, the detector sensor operates by the Van der Waal's absorption forces of the unknown contaminant on a conductive material, such as carbon black graphite or metallic particles, which had been embedded in a resilient base member, such as polyethylene or silicone rubber. The absorption forces of the unknown contaminant on the surface of the conductive particles pulled them apart and caused the resistance of the detector to change. This resistivity change was used to indicate the presence of the unknown and its identity by correlating the resistivity change with the Van der Waal's constant of the unknown gas.
Commercially available devices, embodying the principles of the earliest Dolan et al patent, exhibit a tendency to load up causing the base line reading to shift. In addition, as the particles fall out of the resilient base material, the base line reading also shifts. The more recent Dolan et al patents require the device to be operated in a non-linear voltage range or in a current saturated mode to exhibit enhanced sensitivity. In view of variation between detectors, these ranges or modes must be determined on an individual basis for each detector. This is time-consuming and costly. Furthermore, these ranges or modes will still be subject to base line variations as particles fall out of the detector or they become contaminated.
Thus, it would be desirable to have a detector sensor and detector which can be operated outside a current saturated regime and/or not within the non-linear voltage range. It would also be desirable to have a detector sensor which will not exhibit a base line drift because of the loss of conductive particles during the detector's operation. In addition, it would be desirable to have a detector sensor and detector which does not require a combustible energy source or high temperature to operate around the volatile and hazardous materials to be sensed. Furthermore, it would be desirable to have a detector sensor which would not expose the sensing particles directly to the unknown contaminant which might poison them.